hugetlb: chg cannot become less than 0

chg is unsigned, so it cannot be less than 0.

Also, since region_chg returns long, let vma_needs_reservation() forward
this to alloc_huge_page().  Store it as long as well.  all callers cast it
to long anyway.

Signed-off-by: Roel Kluin <roel.kluin@gmail.com>
Cc: Andy Whitcroft <apw@shadowen.org>
Cc: Mel Gorman <mel@csn.ul.ie>
Cc: Adam Litke <agl@us.ibm.com>
Cc: Johannes Weiner <hannes@saeurebad.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>